<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  
  <script>

    function balance(a,b,c){
      a=Number(prompt('请输入银行卡余额'))
      b=Number(prompt('请输入当月食宿消费金额'))
      c=Number(prompt('请输入当月生活消费金额'))
      let bal=a-b-c
      alert(`当月银行卡余额为${bal}`) 
        return bal
    }
  
    balance()
  </script>
</body>
</html>